1. Field of the Invention
This invention generally relates to thermocouples for measuring temperatures of hot gases and, more particularly, to sheathed thermocouples having a pressurized interior chamber for atmospherically insulating the encased thermocouple leads and its hot junction.
2. Description of the Prior Art
It is desirable in the glass making industry to measure accurately the temperatures of the gases at different locations in the over-all system of producing glass, particularly the gas temperatures in a glass melting furnace occurring at its ports, regenerators, tunnels, flues, chimney and ejectors. Measured under actual operating conditions the temperatures serve as bases for improvements in operations, modifications in design, and fuel and power savings. As a result, longer furnace life, higher tonnages, improved quality, and lower costs for the production of glass may be achieved.
In a conventional glass melting furnace, fuel is alternately fired, using preheated combustion air, from one side and then the other through a series of ports along each side of the tank at right angles to the flow of molten glass. The raw materials are continually fed in at one end of the tank and molten glass is removed from its other end. The variations and conditions at the various ports down each side of the tank are therefore important in determining the variations in temperature undergone by the raw materials during melting and the glass after melting.
In such glass melting furnace systems, the temperature of the combustion air and exhaust gases may be substantially different from the temperature of their surrounding bodies, and the heat exchanged by radiation between the bodies and thermocouple may be predominant over that by convection. The exchange by radiation from or to the adjacent bodies influence the thermocouple reading so that it may indicate the temperature of such bodies or some temperature in between the bodies and the exhaust gases or combustion air rather than the temperature of the exhaust gases or combustion air.
Thus, in order to measure accurately the temperatures of these exhaust gases or combustion air whose temperatures are different than those of their surroundings, aspirating pyrometers may be employed. Such aspirating pyrometers may include a metal thermocouple tube centered within an aspirated gas stream extending the length of the pyrometer. A metal thermocouple sheath holder may be threadedly connected to the hot end thereof and, in turn, a closed end ceramic thermocouple sheath which extends inside a radiation shield assembly is normally sealed to the holder by a refractory cement.
The importance of protecting the thermocouple from the gases by means of a sheath not only to preserve the thermocouple but also to prevent false readings, is self evident. However, even though the thermocouple is protected by a ceramic sheath, it can progressively lose its electromotive force on account of its being indirectly attacked and contaminated by the hot gases and condensation which may have permeated the sheath, seal and holder, as well as by direct attack if the sheath, seal and holder have deteriorated or broken.
The periodic reversal of firing and gas flow through a glass melting furnace presents a problem as waste combustion gases carry highly corrosive fluxes from the tank, and the pyrometer used for measuring the temperature of the preheated combustion air must be protected from such corrosive gases and condensation, particularly in cases when the aspirated flow is reduced to zero as required for calibration of the pyrometer under operating conditions. These highly corrosive fluxes from the melting tank can pass through the sheath, seal, holder, or housing by reason of their porosity, deterioration, or being broken, and can attack the thermocouple and modify its readings as a result of this, or loss of insulation electrically from effects of condensation or destroy its hot junction.
One attempt to overcome this problem is shown in U.S. Pat. No. 1,615,451, issued Jan. 25, 1927. As shown therein, an air flow path is provided so that atmospheric air can traverse the interior of the thermocouple protector tube and carry off any corrosive gases which have penetrated the protector tube. However, the corrosive gases or condensation still physically contact the thermocouple and can thus attack, cause loss of effective insulation, and destroy the junction thereof. This is true even for the embodiment shown in FIGS. 3 and 4 since the gas can penetrate the inner casing tube H.